The present invention relates to an optical microlithography apparatus more particularly used for producing integrated circuits. It more specifically relates to the local alignment system.
It is known that an integrated circuit is produced by a succession of depositions of layers and the etching of said layers. In order to produce a pattern or motif by etching, the integrated circuit is covered with a photosensitive resin layer and said resin is irradiated with a high-energy beam, which is intercepted by a mask, which reproduces the image of the pattern to be produced in the integrated circuit. The irradiated resin is then removed, so that areas of the integrated circuit which are to be etched are left bare.
In the case of circuits with a minimum size of the pattern (width of metal conductors, length of MOS transistor channels, etc.) of roughly at least 3 .mu.m, the most widely used method is to produce a scale 1 mask. This method, called 1:1 projection, is of interest because of its low cost and simplicity permitting a high production rate.
This simple method cannot be used for producing integrated circuits, whereof the typical pattern dimension is approximately 1 .mu.m. Such circuits are mainly produced by two different methods, namely optical microlithography and electron beam microlithography.
The apparatus according to the invention is based on the first method. More specifically, it utilizes the direct step or wafer method, in which the ratio of the respective dimensions of the mask and an integrated circuit is approximately 10:1, so that a photoreduction lens is placed between them.
In order to successively position each integrated circuit of a wafer beneath the mask with a view to irradiating the same, the apparatus comprises a wafer displacement means. The integrated circuits are regularly arranged on the wafer, and said means brings about a displacement of the wafer by a constant distance between two irradiations. The displacement length can e.g. be controlled by a method utilizing the interference of laser beams.
Two known methods are used for aligning the integrated circuits of a wafer in the optical axis of the apparatus, and these two methods will be described with reference to FIGS. 1 and 2. These methods are respectively the overall alignment method and the local alignment method.
FIGS. 1a, 1b and 1c illustrate the overall alignment method of a wafer. FIG. 1a is a plan view of a wafer 2 having a plurality of rectangular zones 4, regularly positioned over the wafer. Successive layers will be deposited on said zones and then etched, so as to produce an integrated circuit. Throughout the remainder of the present text, these zones will be called chips.
Apart from these chips, wafer 2 also has position finding marks 6, which are used for aligning the wafer. These marks can be formed by a single line, as shown in the drawing, or can be two-dimensional, e.g. in the form of a cross. In the first case, at least two marks are necessary for positioning the wafer in translation and in rotation, whilst in the second, a single mark is sufficient.
The wafer is aligned by an optical system, diagrammatically indicated by FIG. 1b. The optical system shown comprises two corner mirrors 8, a prism 10 and an alignment microscope 12. This optical system is connected to a means 23 (FIG. 3) controlling the displacement of the wafer support in such a way that the position finding marks occupy reference positions.
The wafer is then translated, so as to be positioned beneath the optical column of the microlithography system. This optical system comprises a monochromatic light source S, a convergent lens 14, a reticle 16 constituted by a glass plate 18, on whose lower face is formed a mask 20 by deposition and then etching of a metal sheet, and a photoreduction lens 22. Each chip 4 of wafer 2 is successively irradiated.
The correct pre-irradiation of alignment of a chip 4 before irradiation is not checked for all chips. This saves time and consequently leads to a high production rate, but also leads to a significant tolerance in the alignment of the different layers forming the chip. In the case of known machines, this alignment offset is approximately 0.7 .mu.m, and is largely due to the displacement of the wafer from the alignment system to the optical microlithography system, said displacement being roughly 100 mm long.
This 0.7 .mu.m tolerance in the alignment of the chips makes it impossible to produce patterns with a size less than 2 .mu.m. For finer or more detailed lithography, it is necessary to use an apparatus equipped with a local alignment system, i.e. a system making it possible to realign each chip of the wafer prior to irradiation. Such an apparatus is diagrammatically shown in FIGS. 2a and 2b.
FIG. 2a shows in plan view a wafer 2 with a plurality of chips 4. A position finding mark 24 is placed alongside each chip. These marks make it possible to individually align each chip beneath the optical microlithography system. These marks can have one dimension, e.g. a line, or two dimensions, e.g. a cross.
The optical microlithography system of FIG. 2b has the same elements as that of FIG. 1c, i.e. a monochromatic source 8, a convergent lens 14, a reticle 16 constituted by a glass plate 18 and a mask 20 as well as a lens 22.
A reference mark 26 has been added to plate 18 alongside mask 20. This mark is projected by photoreduction lens 22 on to wafer 2. The correct alignment of a chip of the wafer is brought about by making the projection of the mark 26 coincide with the position finding mark 24, of the chip positioned beneath the photoreduction lens. The image of mark 26 on wafer 2 and the surrounding area is reflected by a beam splitter 28 positioned between reticle 16 and the photoreduction lens 22 for analysis purposes. A displacement means then controls the movement of the wafer, as a function of signal 29, so as to correctly align the chip 4 to be irradiated.
The local alignment permits a better accuracy in the superimposing of the different layers of a chip. The alignment precision is in this case approximately 0.1 .mu.m, which is much better than that in the apparatuses equipped with an overall alignment system. It is therefore possible to produce integrated circuits, whereof the minimum size of the patterns is approximately 1 .mu.m.
It is clear that the alignment of each chip is prejudicial to the production rate of the integrated circuit compared with overall alignment. In order not to excessively limit this production rate, realignment only takes place every n chips and not every chip, the value of n being chosen as a function of the characteristics of the apparatus.
The local alignment system of the apparatus shown in FIG. 2b is not perfect. A first limitation of this system results from the fact that the photoreduction level 22 is only corrected for a specific wavelength, which is that of the monochromatic source S. Thus, a correct alignment cannot be obtained with said source. This is prejudicial, because the position finding mark 24 is located in the field of the photoreduction lens 22 and is consequently subject to the action of the radiation from source S.
In addition, according to the technological levels, the position finding mark 24 does not have a very good contrast, which makes the alignment of the associated chip difficult.
The local alignment system of the apparatus according to the invention obviates these problems by permitting the illuminating of mark 24 by a wide spectrum optical beam, or by the choice of a monochromatic beam, whose wavelength can be chose from among a large number thereof.
Among the available lighting sources, it is therefore advantageously possible to use a wavelength, to which the resin covering the chip is insensitive.
The photoreduction lens, which is free from aberrations for a single wavelength, is not compatible with the lighting source according to the invention. Moreover, a photoreduction lens free from aberrations for one wavelength range is substantially impossible to produce with the present state of the art.
It is therefore proposed to arrange a beam splitter between the photoreduction lens and the wafer in order to illuminate the latter, without utilizing the mean s of the optical microlithography system.
This beam splitter also has the function of reflecting the image of the area surrounding the position finding mark of the chip to be irradiated said image then being processed by a means controlling the displacement of the wafer in order to bring about the alignment of the chip to be irradiated.